1. Field of the Invention
This invention relates to a process of producing silver halide grains and to apparatus therefor. More particularly, this invention relates to a process of producing silver halide grains which exhibit a narrow grain size distribution and to apparatus therefor.
2. Description of the Prior Art
Silver halide grains are ordinarily formed by the double decomposition reaction of a water soluble silver salt solution and a water soluble halide solution. Processes of making silver halide grains which exhibit narrow grain size distribution are disclosed in U.S. Pat. Nos. 3,415,650, 3,692,283 and British Patent No. 1,323,464.
In the method disclosed in U.S. Pat. No. 3,415,650, as illustrated in FIGS. 1-2, there is provided an arrangement wherein a rotating hollow mixing device 3 (whose interior is filled with an aqueous colloid solution) having openings such as slits (hereinafter referred to as "slits") 2 in the middle part of an expanded cylindrical wall as shown in FIG. 2 is disposed in a reaction vessel 1 and provided with a rotating vertical shaft. The reaction vessel 1 contains the aqueous colloid solution as shown in FIG. 1. A water soluble silver salt solution and a water soluble halide solution are introduced from open ends at upper and lower portions of the mixing device through conduits 4, 4' into the mixing device 3 while the mixing device is rotated at high speed to cause the same to undergo rapid mixing and reaction and to form small silver halide grains. The thus formed small silver halide grains are then expelled through slits 2 in the wall of the mixing device 3 into the aqueous colloid solution (hereinafter referred to as the "bulk liquid") outside of the mixing device 3 due to the centrifugal force produced by the rotation of the mixing device 3 and the grains are ripened. Although not specifically described, it is presumed that when the silver halide grains expelled into the bulk liquid are again circulated within the mixing device 3, the growth of the silver halide grains occurs or continues. Hence, circulation of the bulk liquid (as well as the silver halide grains suspended in the bulk liquid) occurs and is maintained by the suction force in the upper and lower open ends of the rotating mixing device 3, the suction force being produced by the discharge of the silver halide grains from the mixing device.
However, the above method has several disadvantages. First, very large grains tend to be formed since high concentration reaction liquids are reacted with one another; second, since the degree of mixing within the mixing device and the circulation of the bulk liquid are both dependant on the rotation of the mixing device, the parameters thereof cannot be varied independently; third, the rotating mixing device is of complicated construction, resulting in difficulty of washing the apparatus; and, fourth, the downward flow toward the mixing device 3 tends to drag air therein to produce bubbles, resulting in uneven reaction and the necessity of removal of the bubbles thereafter in the process.
The method disclosed in British patent specification No. 1,323,464 relates to an improvement in the method disclosed in U.S. Pat. No. 3,415,650. As shown in FIG. 3, a mixing device 3 is housed in a casing 6 which is securely fixed thereto, mixing device 3 having outlet ports 5 in its wall and being divided by a disc 7 into upper and lower chambers 8 and 8'. Mixing device 3 and the casing 6 are filled with an aqueous colloid solution as earlier described. A water soluble silver salt and a water soluble halide are fed into chambers 8, 8', through conduits 4, 4', respectively, of the mixing device 3 and are immediately diluted by the aqueous colloid solution in chambers 8, 8' and discharged from outlet slits 2 of the mixing device into the casing 6 by rotation of the mixing device 3 and thence into the bulk liquid outside the casing 6 from the outlet ports 5 in outlet tubes 9 of the casing 6. In this manner, small silver halide grains formed by mixture of reaction liquids discharged into the bulk liquid are again carried into the mixing device 3 and further grown by circulation of the bulk liquid resulting from rotation of the rotating mixing device 3. This circulation of bulk liquid occurs and is maintained by the suction force in the upper and lower ends of the rotating mixing device 3 in a manner similar to that described in U.S. Pat. No. 3,415,650.
The method of this British Patent has the feature that since reacting liquids are individually diluted and thereafter reacted, very large silver halide grains form only with difficulty, even if the concentration of the reacting liquid supplied is raised, whereby the first disadvantage encountered in the method disclosed in the U.S. Pat. No. 3,415,650 can be eliminated. However, the fact that the degree of mixing and the circulation amount of the bulk liquid are both determined only by rotation of the mixing device still remains as a fault, and hence, the above second disadvantage is not eliminated. Moreover, in this method, the mixing device is divided into upper and lower chambers so that the reacting liquids can individually be diluted, and a casing is provided therearound, resulting in an extremely complicated construction, and, as a result, in the event a system needs to be washed when a material is changed-over after operation has been completed, washing is extremely difficult. Thus, the above-mentioned third disadvantage still remains unsolved. Furthermore, the downward flow toward the mixing device 3 is still present and, therefore, the above-mentioned fourth disadvantage still remain unimproved.
Further, in the method disclosed in U.S. Pat. No. 3,692,283, there is provided a centrally expanded cylindrical hollow rotating mixing device 3 formed with slits 2 in the upper wall thereof located adjacent the bottom surface of a reaction vessel 1 containing an aqueous colloid solution therein, as shown in FIG. 4, to effect reaction to form silver halide grains. According to this arrangement, the silver salt solution and halide solution are supplied from the lower portion of the rotating mixing device 3 to the lower zone of the mixing device 3, passing through conduits 4, 4', while the rotating mixing device 3 is rotated to discharge silver halide grains formed therein along with aqueous colloid solution outward through the slits 2 so that both reaction liquids are sucked into the rotating mixing device 3 to receive vigorous mixing caused by rotation of the mixing device 3 and are quickly reacted to form silver halide grains. The resulting silver halide grains are expelled outward from the mixing device 3 through the slits formed in the upper wall of the mixing device 3 and join with the circulating flow of bulk liquid which results from the rotation of the mixing device 3 and are subsequently recycled into the mixing device 3 for further growth as silver halide grains. The above-mentioned method improves the fourth disadvantage among the four disadvantages noted for U.S. Pat. No. 3,415,650, but the other three disadvantages still remain unsolved.